This invention is directed to a process for cathodically electrodeposition a clear topcoat over one or more conductive paint layers which have been deposited by an electrodeposition process or by conventional spray application methods onto a conductive metal or another conductive surface.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. The electrodeposition of primers to metal substrates used for auto and truck bodies is widely used in the automotive industry. In the electrodeposition of primers, a conductive article, such as an autobody or auto part, is immersed in a bath of a primer composition of an aqueous emulsion of film forming polymer and acts as one electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired thickness of coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and counter-electrode is the anode.
Resin compositions or polymers used in the bath of a typical cathodic electrodeposition process are also well known in the art. These resins are typically polyepoxides which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through a reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating deposited on the article is a function of the bath characteristics, the electrical operating characteristics, the immersion time and the like.
The coated article is removed from the bath after a given period of time and is rinsed with deionized water. The coating is cured typically in an oven at sufficient temperature to produce a crosslinked coating.
Cathodic electrocoating compositions, resins, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979: Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 28, 1984 which are incorporated herein by reference.
Cathodic electrodeposition is widely used in the automotive industry and other industries because it provides a finish that gives superior corrosion protection, covers recessed or hard to reach areas, deposits a uniform film thickness free of voids and defects such as sags or runs, is not labor intensive, has less emissions to the environment and provides for almost 100% material use.
It would be desirable to apply a clear topcoat by electrodeposition, over one or more previously applied paint layers because of the many advantages electrodeposition has over conventional spray applied clear topcoats, such as, improved edge coverage, uniform coatings (free of sags and runs), fully automated operation, low volatile organic content, near 100% material use efficiency, no overspray, additional corrosion protection to recessed areas and interior cavities However, for the electrodeposition process to be operative for the application of the clear coat, the previously applied paint layer must be electrically conductive. This has presented a particular problem since pigments that provide electrical conductivity are colored such as carbon black and iron oxide pigments making it difficult and often impossible to have very light colors such as white or beige of the previously applied paint layer.